[Linrad] Re: system gain

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hi Dave,

> The antenna, preamp, power amp and transverter are mounted remotely at 
> the antenna. The system nf and gain to the IF is quite good, say .7 dB 
> nf and 25 dB gain.
OK. 

> From there, the 144mhz IF travels through about 7 dB 
> of loss to a 3 dB splitter.
Now we have 10 dB loss in total.

> One side of the splitter goes to the IF 
> radio,
Let me guess that the noise figure of the IF radio is 5 dB.
That would give a noise figure at the other end of the
cable of 15 dB. The composite system noise figure would 
be 1.04 dB

> the other side to the .6 dB nf, 18 dB amp, and then into the 
> SDR-14 and on to the computer.
At the antenna it looks like 10.6 dB NF and 8 dB gain. The composite
noise figure becomes 0.82 dB with an associated gain of 33 dB.
This is then connected to the SDR-14 direct input with a noise
figure of 37 dB. The compsite noise figure for your Linrad
part would be 5.7 dB. Totally unacceptable.....

You should use the 0.1 to 30 MHz input which has a noise figure
of about 17 dB. Then your system noise figure would be 0.91 dB.
Probably just a little better than your IF radio.

If I were you, I would move the 0.6 dB NF amplifier to the other 
side of the splitter and possibly insert an attenuator in front
of the IF radio. The system noise figure for Linrad would then be 
0.84 dB for Linrad and the same for your IF radio if the attenuator
is selected to provede a NF of 17 dB.

73

Leif / SM5BSZ
 
 






> 
> The IF radio hears signals better than the LINRAD system displays them. 
> My whole point is to bring the LINRAD system up to or better than the 
> sensitivity of the IF radio. Of course this is a better problem to have 
> than the other way around--I think :-).
> 
> I hope this helps you visualize my system. I don't believe the problem 
> is at the remote site since both the IF radio and the LINRAD system see 
> the same signal. The question is how to tweak the LINRAD parameters to 
> bring its performance up to that of the IF radio.
> 
> Thanks for your thoughts.
> 
> 73,
> 
> Dave, K4TO
> 
> radio1@xxxxxxxxxxxxx wrote:
> > In the old days, when every thing was analog, we would say that the 
> > system noise figure was set by the first amplifier's noise figure.  Assuming 
> > you have enough gain.  From the first amplifier noise is just added.  I can't 
> > visulize your system do you have any gain near the antenna?  The losses 
> > introduced by coax also adds to the system noise if the amplifier is after 
> > the line loss.
> > 
> > Maybe this will help.
> > John K1VBM
> > 
> > 
> > ---- Original Message ----
> > 
> > From: Dave Sublette <k4to@xxxxxxxx>
> > To: linrad@xxxxxxxxxxxxxxxx
> > Cc: 
> > Subject: [Linrad] system gain
> > Date: Wed, April 08, 2009, 10:11:00
> > 
> > 
> > Good morning,
> > 
> > I have been watching the email thread on setting optimum gain and other 
> > settings for the LINRAD program. I have used the program, rather 
> > casually, for two years or so to look at weak signals on the 144 mhz IF 
> > outputs from my microwave transverters. I put a gain stage ahead of my 
> > SDR-14's direct input and that stage measures at 0.9 dB nf and 18 dB 
> > gain or so at 144 mhz.
> > 
> > When I remove this amplifier from the system, the noise floor on the 
> > main spectrum display on LINRAD drops 8 dB. I read in the emails that it 
> > should drop 15 dB, if I understand things correctly. I am looking at 
> > about a 160-170 Khz slice of the spectrum around 144.1 Mhz,
> > 
> > I do note that the visual display on Linrad is not as sensitive as my 
> > receiver on a splitter from the same IF.
> > 
> > My question is,  "Where should I look next to either find the problem or 
> > adjust the LINRAD parameters to bring the LINRAD system sensitivity up 
> > to standard?"
> > 
> > I continue to read the web page and previous emails from VK2DU on 
> > setup. 
> >   Nothing I have tried seems to be working. I would list all my settings 
> > here, but perhaps that would be premature.
> > 
> > Any advice to be had out there?
> > 
> > Thanks,
> > 
> > Dave, K4TO
> > 
> > 
> > 
> > 
> > 
> > > 
> > 
> 
> > 

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